1. Field of the Invention
This invention relates to a device for measuring light incident on an optical system. More particularly, the invention is concerned with such a device which is adaptable to a light metering device in a photographic camera.
2. Description of the Prior Art
The light metering device in a photographic camera splits a portion of an image-forming light beam which has passed through an objective lens by a beam splitter, and the thus split light is detected by a photo detector. For the light beam splitter useful for such light metering device, there has so far been used a single half mirror which is obliquely disposed in the camera. Such a beam splitting device for use in a camera, etc. should desirably be as thin as possible. U.S. Pat. No. 3,464,337 discloses a device in which two members, each being provided on its surface with a plurality of slant reflecting surfaces, are cemented together to form a prismatic line raster, and a portion of the incident light is split for light measuring.
In the case of a light measuring device used in a single lens reflex camera or a similar camera, the light measuring device is desired to have the ability to measure light in proper correspondence to the distribution of brightness on an object. U.S. Pat. Nos. 4,103,153 and 4,178,084 have disclosed such a light measuring device in which a hologram has been used as beam splitter. The hologram is formed making use of the interference phenomenon of light. In this case, the above object is attained by superposing two or more holograms one on another or by making the hologram have a certain distribution of intensity in the interference fringe.